1. Field of the Invention
The invention relates generally to the field of seismic surveying. More particularly, the invention relates to methods for planning and performing seismic surveys.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the earth's surface. Seismic surveys are performed by generating seismic signals at source locations and receiving the resulting seismic signals at receiver locations. The seismic signal emanates from the source, and spreads outwardly in a substantially spherical pattern. When the signal reaches interfaces in the subsurface between strata having different acoustic velocities (and also different acoustic impedances), a portion of the energy will be reflected from the interface, and a portion of the energy will be transmitted through the interface. For the reflected energy the raypath follows the rule that the angle of incidence is equal to the angle of reflection. The raypath of the energy traveling through the interface will be altered according to Snell's Law.
If the earth's surface and all reflecting interfaces in the subsurface are substantially flat and parallel to the earth's surface, then for seismic signals generated at a given source location and detected at a given receiver location, it can be assumed that the reflection locations will be directly below the mid-point between the source location and the receiver location. Accordingly, if seismic signals are generated at an evenly spaced pattern of source locations, such as a rectangular grid of locations having uniform spacing of the source locations in the in-line direction and in the cross-line direction, and if the receiver locations are also evenly spaced, the locations at which the resulting seismic signals are reflected from a given subsurface reflecting interface will also be evenly spaced, and the reflecting interface will be uniformly illuminated. As used herein the term “illumination” refers to the quantity of signals reflected from a designated area of a subsurface reflecting interface and detected by the receivers.
Frequently, subsurface structures that are of interest to the explorationist are neither parallel to the earth's surface nor flat. Because of the irregularities in the structure of this body, the seismic raypaths emanating from the surface seismic sources that travel to this body and are then reflected to the earth's surface will reach the earth's surface in a very irregular pattern, so that the signals received by the receivers positioned in a standard regular grid pattern as described above represent an illumination of the body which has substantial variation from one part of the structure to another.
For areas of low illumination, the “fold”, i.e., the number of received seismic data traces representative of the areas of low illumination may be inadequate to satisfactorily map such locations. Accordingly, it is an object of this invention to structure a seismic survey that enhances the illumination of subsurface areas of interest that would otherwise have low illumination.